diff --git a/python_multipart/multipart.py b/python_multipart/multipart.py
index d50e5b3..5f0dfe8 100644
--- a/python_multipart/multipart.py
+++ b/python_multipart/multipart.py
@@ -409,8 +409,10 @@ def __init__(
             # Extract just the basename to avoid directory traversal
             basename = os.path.basename(file_name)
             base, ext = os.path.splitext(basename)
-            self._file_base = base
-            self._ext = ext
+        else:
+            base, ext = b"", b""
+        self._file_base = base
+        self._ext = ext
 
     @property
     def field_name(self) -> bytes | None:
diff --git a/tests/test_file.py b/tests/test_file.py
index a2aa134..562d2f1 100644
--- a/tests/test_file.py
+++ b/tests/test_file.py
@@ -3,6 +3,32 @@
 from python_multipart.multipart import File
 
 
+def test_none_file_name_with_keep_extensions_no_attribute_error() -> None:
+    """File(file_name=None) must not raise AttributeError when flushed to disk
+    with UPLOAD_KEEP_EXTENSIONS=True (regression: _ext was not initialised for
+    the None branch)."""
+    file = File(None, config={"UPLOAD_KEEP_EXTENSIONS": True, "MAX_MEMORY_FILE_SIZE": 0})
+    try:
+        # Writing one byte exceeds MAX_MEMORY_FILE_SIZE=0, triggering flush_to_disk()
+        # which previously accessed self._ext -> AttributeError.
+        file.write(b"x")
+    finally:
+        file.close()
+
+
+def test_none_file_name_with_keep_filename_no_attribute_error() -> None:
+    """File(file_name=None) must not raise AttributeError when flushed to disk
+    with UPLOAD_KEEP_FILENAME=True (regression: _file_base was not initialised
+    for the None branch)."""
+    file = File(None, config={"UPLOAD_KEEP_FILENAME": True, "MAX_MEMORY_FILE_SIZE": 0})
+    try:
+        # This reaches the UPLOAD_DIR-is-None branch, which accesses self._ext
+        # via the suffix assignment — previously an AttributeError.
+        file.write(b"x")
+    finally:
+        file.close()
+
+
 def test_upload_dir_with_leading_slash_in_filename(tmp_path: Path) -> None:
     upload_dir = tmp_path / "upload"
     upload_dir.mkdir()